Although molecules, which contain different isotopes of a given element, differ from each other only in respect of their fine structure, it is well known that it is often possible selectively to excite those molecules containing a given isotope, by the use of the almost purely monochromatic radiation of a laser. The wavelength of the laser radiation is suitably chosen for excitation of the molecules containing a given isotope, in some cases even to the extent of ionisation of those molecules, whilst other molecules containing a different isotope do not effectively absorb the radiation and are therefore not perceptibly excited (see for example "Scientific American", February 1977, pages 86 to 98).
The excited molecules can be separated by physical or chemical methods, the latter having the advantage of simpler execution and potentially higher yield.
For chemical separation the most diverse reactions have been employed, such as unimolecular reactions involving the predissociation and dissociation of molecules, e.g. the selective photo-induced dissociation of the tetrazine molecule EQU C.sub.2 H.sub.2 N.sub.4 into N.sub.2 and HCN,
or exchange reactions, wherein an isotope-selective excited molecule is brought into reaction with a coreactant, referred to as a scavenger.
The utility of unimolecular reactions is materially restricted because only relatively small molecules can be employed, which nevertheless exhibit an acceptable isotope displacement in the spectrum. Known exchange reactions have the disadvantage that the resulting transitionally produced molecules cause undesirable side reactions (referred to as radical scrambling), whereby the enrichment factor can be considerably impaired.